Electrolytic hole forming cathode electrode



Nov. 15, 1966 R. T. HALLSWORTH ETAL 3,235,844

ELECTROLYTIC HOLE FORMING CATHODE ELECTRODE Original Filed May 31, 1962 Attorneys United States Patent 3,285,844 ELECTROLYTIC HOLE FORMING CATHODE ELECTRODE Raymond Thomas Hallsworth, Earby, and Irvin Holroyd,

Barnoldswick, England, assignors to Rolls-Royce Limited, Derby, Derbyshire, England, a company of Great Britain Original application May 31, 1962, Ser. No. 199,092. Divided and this application Nov. 19, 1962, Ser. No. 238,387 Claims priority, application Great Britain, June 5, 1961, 20,259/ 61 3 Claims. (Cl. 204284) This application relates to an electrode for use as a cathode electrode in apparatus for forming holes electrolytically in a metallic anode workpiece, and is a divisional application from our patent application Serial No. 199,092 filed May 31, 1962, now abandoned.

Where it is desired to form holes of accurate dimensions by an electrolytic process, the cathode electrode used to form the hole preferably has an external insulating coating to prevent sideways cutting by the electrode. With such an electrode, electrolyte may be supplied through a pasage in the electrode, the electrolyte escaping through a gap between the external surface of the insulating coating and the internal surface of the hole being formed. This gap may be very small, and any pores in the insulating coating will permit sideways cutting by the electrode, the amount of such cutting depending in part on the size of said gap. It is therefore very important that the insulating coating should be as non-porous as possible, and preferably completely non-porous. Furthermore the pressure and rate of flow of the electrolyte in said gap may be very high, so that the insulating coating must be sufiiciently strong to withstand the resulting physical forces acting on it. A common fault with such electrodes is peeling or separation of the insulating coating adjacent the uninsulated working tip of the electrode. This can be avoided by using a strong durable material for the insulating coating. Unfortunately, however, insulating materials which are strong and durable are generally rather porous, whereas highly non-porous insulating materials are generally not particularly durable and strong. It should be understood that where small holes are to be formed, the maximum thickness of the insulating coating is limited.

It is one object of the invention to provide an electrode having a non-porous polyvinylchloride insulating coating. Such coating may for example comprise a composite coating of plastisol on plastisol primer bonded to the external surface of the electrode.

Another object of the invention is to provide an electrode with a first strong durable insulating coating adjacent the working tip of the electrode, and a second highly non-porous insulating coating made of different material from the first coating, the second coating forming a rearward continuation of the first coating.

Another object of the invention is to provide an electrode with a forward outwardly projecting flange from whose rearward face the insulating coating extends rearwardly, the insulating coating immediately adjacent said flange having the same transverse external dimensions as the flange, so that the flange affords some protection for the insulating coating.

' These and other objects of the invention will be apparent from the following description of a preferred embodiment of the invention, given by way of example, and shown in the accompanying drawing which is a transverse cross-sectional view of an electrode according to the in" vention.

The electrode has a circular transverse cross-section "ice and comprises a hollow stem 81 made of stainless steel. The stem has an annular groove 82, and terminates in an enlarged flange 83 forming an uninsulatedworking tip. A coating 84 of the phenolic epoxy resin Araldite (which is a very strong adhesive) is located in the groove 82, and the stem 81 is provided with a composite coating of plastisol primer 85 and plastisol '86, the layers of which partially extend over the coating 84. The axial length of the uninsulated rim of the working tip 83 is kept as short as possible in order to reduce sideways cutting by the electrode.

A satisfactory method of coating the electrode with the plastisol primer and plastisol, i.e. after the coating 84 has been formed on the electrode, is as follows:

Firstly, the ends of the electrode are sealed, and the electrode is degreased. The electrode is then etched, using a solution of 10% Fe(NO 9.H O in concentrated HCl, for 1 /2 minutes at a temperature of 60 C. The etching leaves a black layer on the electrode, which is brushed off and the electrode is washed thoroughly and dried in an oven at C. for about 15 minutes. The electrode is then coated with plastisol primer 219P.X. by partially suspending the electrode in a vessel containing the primer, and then draining the primer from the bottom of the vessel. In order to cure the primer, the electrode is air dried for 20 minutes, and is then further dried at a temperature of C. for 20 minutes. In this way a good bond is formed between the primer 85, the coating 84 and the stem 81. The electrode is then coated with plastisol 218 by slowly withdrawing the electrode from a tank containing the plastisol and in which the electrode is initially partially immersed. The rate of withdrawal of the electrode to obtain the desired uniformity and thickness of the coating, depends on the length and diameter of the electrode. The electrode is then placed in an evacuated chamber in order to remove bubbles in the plastisol coating, and the plastisol is then cured at C. for one hour. In this way a good bond is formed between the plastisol 86, the primer 85 and the coating 84.

The finished electrode should be inspected to ensure that the thickness of the composite coating is 0.003 to 0.004", and is free from lumps and does not taper by more than 0.002" from end to end. The coating is then checked for porosity by immersing the electrode and a metal plate in a solution of saturated brine and making the electrode 25 volts negative relative to the metal plate. The presence of even a very small pore in the coating is revealed by a stream of gas bubbles. An optical inspection is also carried out to detect any other imperfections e.g. dust particles in the coating, which may be likely to cause failure during operation.

We claim:

1. An electrode for use as a cathode electrode in apparauts for forming holes electrolytically in a metallic anode workpiece, comprising a hollow elongate metallic member having an uninsulated forward working tip, an intermediate portion adjacent said Working tip, and a rear portion, the intermediate portion having a first external insulating coating extending rearwardly from said working tip, and said rear portion having a second. external insulating coating forming a continuation of said first coating, said second coating comprising a non-porous composite coating of polyvinylchloride bonded to the external surface of said rear portion, and said first coating being made of strong, durable epoxy resin material.

2. An electrode as claimed in claim 1 in which both the rear portion and the intermediate portion have smaller external transverse dimensions than the working tip and the external transverse dimensions of the first coating at the forward end thereof are the same as those of the working tip.

References Cited by the Examiner UNITED STATES PATENTS 2/1964 Williams 204224 7/1965 Williams 204-143 JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner. 

1. AN ELECTRODE FOR USE AS A CATHODE ELECTRODE IN APPARATUS FOR FORMING HOLES ELECTROLYTICALLY IN A METALLIC ANODE WORKPIECE, COMPRISING A HOLLOW ELONGATE METALLIC MEMBER HAVING AN UNINSULATED FORWARD WORKING TIP, AN INTERMEDIATE PORTION ADJACENT SAID WORKING TIP, AND A REAR PORTION, THE INTERMEDIATE PORTION HAVING A FIRST EXTERNAL INSULATING COATING EXTENDING REARWARDLY FROM SAID WORKING TIP, AND SAID REAR PORTION HAVING A SECOND EXTERNAL INSULATING COATING FORMING A CONTINUATION OF SAID FIRST COATING, SAID SECOND COATING COMPRISING A NON-POROUS COMPOSITE COATING OF POLYVINYLCHLORIDE BONDED TO THE EXTERNAL SURFACE OF SAID REAR PORTION, AND SAID FIRST COATING BEING MADE OF STRONG, DURABLE EPOXY RESIN MATERIAL. 